The present invention relates to a phase modulator for a radio beacon which produces a specific phase modulation required for such an application.
The conventional approach to produce phase modulation in position indicating radio beacon (PIRB) devices which send signals to search and rescue satellite aided tracking (SARSAT) satellites orbiting around the earth is to perform analog phase modulation. The requirement is for phase modulation between the phase angles of +1.1, 0 and -1.1 radians. The data must cause the carrier to dwell for specific intervals at each phase angle and also to transition smoothly and progressively between the primary phase angles at a defined rate. This is a difficult requirement to meet using analog phase modulation circuits. Such methods are difficult to reproduce in manufacturing and need careful tuning and periodic re-tuning to counteract ageing. They are also sensitive to temperature variations.
Accordingly, it is an object of the invention to provide an improved phase modulator for a position indicating radio beacon (PIRB). It is a further object to provide such a phase modulator which requires no tuning or re-adjustment. Yet another object of the invention is to provide a phase modulator for a PIRB which is smaller than conventional ones.